Polymer 52 (2011) 1495e1503
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Monomer/modified nanosilica systems: Photopolymerization kinetics and composite characterization Mariola Sadej-Bajerlain a, Hubert Gojzewski b, c, Ewa Andrzejewska a, * a
Faculty of Chemical Technology, Poznan University of Technology, pl. Marii Sklodowskiej-Curie 2, 60-965 Poznan, Poland Institute of Physics, Poznan University of Technology, ul. Nieszawska 13A, 60-965 Poznan, Poland c Max Planck Institute for Polymer Research, Ackermannweg 10, 55-128 Mainz, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2010 Received in revised form 16 December 2010 Accepted 30 January 2011 Available online 24 February 2011
The article describes the influence of the amount and type of organically modified nanosilica (surface and structure modified Aerosil 7200 and surface-modified Aerosil R711) on the photopolymerization kinetics of tetraethylene glycol dimethacrylate and on the physical properties of nanosilica dispersions in the monomer and the polymer matrix. Kinetic measurements showed that silica addition can accelerate or retard the polymerization depending on the silica content; the magnitude of this affect depends on the type of silica modification and can be associated with stability of silica dispersion (as measured by Zeta potential value). The highest reactivity showed compositions containing 4e5 wt.-% of silica and acceleration of the polymerization seems to result mainly from the increase in the propagation rate coefficient. The composites obtained show a uniform dispersion of nanoparticles within the polymer matrix for the silica content at least several wt.-%. The size of aggregates covered with the polymer layer is between 50 and 150 nm for Aerosil R7200 and 75e300 nm for Aerosil R711. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Nanosilica Photopolymerization Kinetics
1. Introduction In recent years much attention has been focused on development of new inorganic-organic composite materials of prospective use in many areas [1e3]. A nanocomposite can be defined as a material resulting from the association of particles having at least one of their dimensions in the range of a few nanometers, dispersed or organized in a polymer matrix [4], and nanohybrid can be defined as nanocomposite for which there exists a relatively strong chemical bond between the nanoparticles and the macromolecular chains that compose (at least partly) the continuous phase [5]. It is well known that nanocomposites can combine advantages of organic polymers and the inorganic materials, which results in an enhancement of various properties including viscoelastic characteristics, fire resistance [6] and barrier properties [7], resistance to scratching [8,9], abrasion [9e11], as well as other mechanical properties [2,10,11]. From among the inorganic substances, silicon dioxide has become of greatest importance as an active filler of polymers because of its good resistance to heat and electricity, mechanical stability, relatively low costs and high specific surface area [11,12]. Silica has been widely
* Corresponding author. Tel.: þ48 61 6653637; fax: þ48 61 6653649. E-mail address:
[email protected] (E. Andrzejewska). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.01.058
applied in various industries. Besides common plastics and rubber reinforcement, many other potential and practical applications of polymer/silica nanocomposites have been reported: coatings, flameretardant materials, optical devices, electronics photoluminescent conducting film, ultrapermeable reverse-selective membranes, proton exchange membranes, grouting materials, sensors, etc. [13]. An easy way to disperse silica in a polymer is the preparation of the composite “in situ”, by the polymerization of a monomer that contains dispersed silica. Especially useful method is the UV-induced process, which is solvent-free, energy efficient, economical in space and can be realized at ambient temperature with the high speed. These features, along with the spatial and temporal control of the curing, make the photopolymerization an attractive method for generating of high performance materials. Photopolymerization found extensive applications in producing photoactive polymerbased systems used in coating industry, paints or printing inks, adhesives, composite materials, printing plates, photo- and stereolithography, holographic recordings, and dental restorative formulations. UV-irradiation in the presence of a suitable photoinitiator is one of the most efficient methods for the generation of highly crosslinked polymers from multifunctional monomers. Polymerization can be carried out under a wide range of conditions, including varying monomer structure, number and type of reactive functional groups, temperature, atmosphere, irradiation rate and photoinitiator type.
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Methacrylate and acrylate monomers are the most widely used in the light-curable systems due to their high reactivity [14]. There are many reports on UV-curable nanocomposites containing fumed silica, but they focus mainly on the mechanical properties of the nanocomposites. It has been shown for instance, that nanocomposite materials reinforced by silica nanoparticles exhibit enhanced scratch and abrasion resistance compared to nanosilica-free composites [15e17]. In the case of surface-modified pyrogenic silica, the comparison with commercially available acrylate suspensions containing colloidal silica revealed a distinct improvement in the surface mechanical properties such as haze and diamond microscratch hardness [9]. The knowledge of the curing kinetics is necessary to understand the process of composite formation and to find technological conditions for the material production. A number of papers considered kinetics of the UV-induced polymerization process of monomer/nanosilica systems, but rather only in general terms [18e26]. Some of papers reported on an accelerating [19,20,23] or decelerating [24] effect of nanosilica addition on the polymerization process and increasing [19,20,23] or decreasing [26] conversion. Explanations considered restriction or facilitation of diffusion of reacting species by nanosilica network depending on the reaction stage [20], inhibition effect of the inorganic network on bimolecular termination [19] or extension of the light path length by light reflection and scattering [23]. There are no detailed reports on the curing kinetics in relation to some physical properties, especially for systems containing fumed nanosilica particles methacrylsilane after-treated and structure modified. Such silicas (in the frame of Aerosil series) are used as modifiers and thickening agents and improve mechanical properties of composites and plastics. Although they are often nanometer sized, their appropriate dispersion to obtain a nanocomposite can be a problem. This work was aimed to give the characteristics of a monomer/ nanosilica system, both before the polymerization (dynamic light scattering (DLS) - particle size distribution, Zeta potential; viscosity), during the UV-induced polymerization (differential scanning calorimetry (DSC) - kinetics) as well as after the polymerization (atomic force microscopy (AFM) and scanning electron microscopy (SEM) - topography and surface morphology) and to show how the amount of nanosilica and type of its modification can influence these characteristics. Tetraethylene glycol dimethacrylate (TtEGDMA) has been used as a model monomer to form the composite matrix. The silicas selected were Aerosil R7200 and Aerosil R711. Aerosil R711 is the fumed silica based on Aerosil 200, treated with 3-methacryloxypropyl-trimethoxysilane to reduce the polarity and provide stronger interaction with the polymer segments - both on physical (surface hydrophobization) and chemical (polymer grafting) ways. Aerosil R711 is used to improve mar and scratch resistance of paints, coatings (e.g. in UV curing systems) and inks. On the other hand, Aerosil R7200 is the active filler with average primary particle size ¼ 12 nm and, according to supplier, substantially improves the scratch resistance of UV-curable coatings. It is also surface treated with 3-methacryl-oxypropyltrimethoxysilane and additionally structurally modified, which reduces the size of agglomerates. This should enable to achieve higher loading levels in liquid systems (up to 20%) with little impact on viscosity [27]. 3-Methacryl-oxypropyl-trimethoxysilane acts as a coupling agent between the inorganic and organic phase. The methacrylic functions present on the surface of both silicas are able to copolymerize with the monomer leading to formation of covalent bonds between modified silica and the polymer, which inhibits macroscopic phase separation between the filler and polymer matrix.
2. Experimental section 2.1. Materials The monomer TtEGDMA was purchased from Aldrich. It was purified from the inhibitor by column chromatography before use. The photoinitiator, 2,2-dimethoxy-2-phenylacetophenone (Irgacure 651) has been kindly donated by Ciba. Aerosil R7200 and Aerosil R711 were gifts from Evonik. The silicas were dried at 110 C for 2 h before use. The monomer/filler mixtures were homogenized by ultrasonication by several hours. The obtained dispersions were slightly cloudy but fully transparent in the photoinitiator absorption wavelength range. 2.2. Methods 2.2.1. Viscosity Viscosities of monomer/silica mixtures were measured at the polymerization temperature (h20 and h40) at 120 rpm with a Brookfield Digital Viscometer model DV-II (cone-and-plate geometry) and with Anton Paar MCR 301 instrument in the plateand-plate geometry at various share rates at 40 C. 2.2.2. Dynamic light scattering Silica suspensions in the monomer were characterized with a Zetasizer Nano ZS (Malvern Instruments Ltd.). Size of silica particles as well as particle size distribution (PSD) were performed employing the technique of non-invasive back scattering method (NIBS) with a constant 173 scattering angle, at (25 0.1) C. The obtained particle size is an intensity-weighted mean diameter, which is also called “z-average diameter”. Zeta potential was measured at (40 0.1) C, i.e. at the temperature of kinetic measurements. 2.2.3. Photopolymerization kinetics Reaction rates (Rp) and conversions (p) were determined by DSC under isothermal conditions at (40 0.01) C in a high-purity argon atmosphere (
